There continues to be an extreme shortage of organs for transplantation. Currently, the major limiting factor in clinical transplantation is the persistent shortage of organs. For example, kidney transplantation is largely dependent upon the availability of organs retrieved from heart-beating cadaver donors. There exists, however, a large and as yet untapped source of organs for transplantation, namely, non-heart-beating cadavers. Non-heart-beating cadavers are accident victims who succumb at the site of an injury and those having short post-trauma survival times. Additionally, non-heart-beating cadavers result when families are emotionally unable to make the decision to donate the organs of a loved one contemporaneously with making the decision to terminate life support. In these situations, the organs are not used because the lack of circulating blood supply (warm ischemia) once the heart stops beating, results in an injury cascade.
An organ marginally, but functionally damaged by warm ischemia cannot tolerate further damage mediated by the hypothermic conditions presently utilized to preserve organs intended for transplantation. Under these conditions, the lipid bilayer experiences a phase-change and becomes gel-like, with greatly reduced fluidity. The essentially frozen lipid in the cell membranes negates the utilization of O2-tension. The metabolic consequence is glycolysis, which is analogous to the state of anoxia. It has been described that below 18° C., hypothermia inhibits the tubular and glomerular activities of the kidney and that at 4° C., the utilization of oxygen is approximately 5% of that at normothermia.
Hypothermic storage can also produce vasospasm and subsequent edema in an organ. Hypothermically preserved organs can experience glomerular endothelial cell swelling and loss of vascular integrity along with tubular necrosis; phenomenon attributable to the hypothermic conditions employed. Hypothermia can also inhibit the Na/K dependent ATPase and result in the loss of the cell volume regulating capacity. The loss of volume regulation is what causes the cellular swelling and damage. An ample supply of oxygen does not actively diminish the amount of this swelling because the cell membrane is essentially frozen, preventing the effective utilization of oxygen. Without adequate oxygen delivery, the anoxia leads to disintegration of the smaller vessels after several hours of perfusion. The lack of oxygen and the subsequent depletion of ATP stores mean that anaerobic glycolysis is the principal source of energy under traditional preservation conditions. The subsequent loss of nucleosides is probably a very important factor in the failure of tissues subjected to warm ischemia and prolonged periods of cold ischemia to regenerate ATP after restoration of the blood supply. The inability to supply adequate oxygen has led to the routine reliance on hypothermia for organ preservation. In the case of warm ischemia, circulatory arrest leads to anoxia where there is no molecular oxygen for oxidative phosphorylation. The lack of molecular oxygen leads to the accumulation of NADH and the depletion of ATP stores with in the mitochondria.
Thus, ischemia (whether warm ischemia or cold ischemia) is an injury cascade of events that can be characterized as a pre-lethal phase, and a lethal phase. The pre-lethal phase produces harmful effects in three ways: hypoxia; malnutrition; and failure to remove toxic metabolic wastes. With the lack of circulating blood comes a lack of molecular oxygen. The resulting hypoxia induces depletion of energy stores such as the depletion of ATP stores in mitochondria. Depletion of ATP leads to cellular changes including edema, loss of normal cellular integrity, and loss of membrane polarity. The cellular changes, induces the lethal phase of ischemia resulting in accumulation of metabolic wastes, activation of proteases, and cell death.
The perfusate solution that represents the current state-of-the-art in hypothermic organ preservation, and provides for optimized organ preservation under hypothermic conditions, contains components which prevent hypothermic induced tissue edema; metabolites which facilitate organ function upon transplantation; anti-oxidants; membrane stabilizers; colloids; ions; and salts (Southard et al., 1990, Transpl. 49:251; and Southhard, 1989, Transpl. Proc. 21:1195. The formulation of this perfusate is designed to preserve the organs by hypothermic induced depression of metabolism. While it minimizes the edema and vasospasm normally encountered during hypothermic storage, it does not provide for the utilization of a substantially expanded donor pool.
This is due to the fact that an organ or tissue damaged by warm ischemia cannot tolerate further damage mediated by the hypothermia. Even with just 30 minutes of ischemia, the post-transplant function of an organ can be compromised. For example, using organs from heart beating cadavers (non-damaged), the immediate non-function rate is estimated to be 25%; within just 30 minutes of warm ischemia, the immediate non-function rate is increased to about 60%. Thus, 60% of the kidneys from non-heart-beating cadavers do not immediately function because of pre-lethal ischemic injury. Further, irreversible ischemic damage and injury is thought to occur to organs deprived of blood flow in just a few hours or less (Klatz et al., U.S. Pat. No. 5,395,314). Unless new sources of organs can be developed, the number of transplantation procedures will remain constant. Additionally, the donor pool cannot be substantially expanded because there is no process/system available to repair pre-lethal ischemic damage in warm ischemically damaged organs or tissues.
Recent efforts have focused on prevention of ischemic damage by intervening with a solution immediately upon cessation of blood flow. For example, a protective solution, disclosed in U.S. Pat. No. 4,415,556, is used during surgical techniques or for organs to be transplanted for preventing ischemic damage to the organ. The protective solution is used as a perfusate to improve aerobic metabolism during the perfusion of the organ. U.S. Pat. No. 5,395,314 describes a method of resuscitating a brain by circulating, after interruption of the blood supply, through the brain a hypothermic preservation solution (approximately 8°-10° C.) designed to lower organ metabolism, deliver oxygen, and inhibit free radical damage.
Although such methods and preservation solutions are useful in preventing ischemic damage in organs, these beneficial effects are overshadowed by practical and functional limitations. First, for such methods and solutions to be effective in preventing ischemic damage, they must be applied immediately (within minutes) after interruption of the blood supply. Logistic restraints, as in the case where an accident victim becomes an organ donor, may severely curtail the use of such methods and solutions. For example, their use is impractical at the site of an accident or in the ambulance where initiation of the ischemic injury cascade would occur. Secondly, irreversible ischemic damage and injury is thought to occur to organs deprived of blood flow in minutes (e.g., brain) or within just a few hours (heart, kidney). An organ or tissue, marginally, but functionally, damaged by warm ischemia cannot tolerate further damage mediated by hypothermic storage prior to transplantation or restoration of blood flow upon transplantation. One reason is that restoration of the circulation after ischemic-reperfusion may paradoxically result in further tissue damage. (McCord et al., 1985, N Engl J Med 312:159-163). During reperfusion, reoxygenation of ischemically damaged tissue can result in further tissue injury caused through the formation of oxygen free radicals, depletion of free radical scavengers, and the release of chemotactic agents.
Thus, there is a need for a system, including a preservation solution useful for initial organ flushing and as a perfusate for in situ or ex vivo preservation of organs for transplantation, which employs a warm preservation technology which minimizes, and, in fact, repairs damage due to warm ischemia, and which supports the organ near normal metabolic rate. Portability and automation of the system is important, particularly in situations where the system is used to initiate organ preservation in situ either prior to or immediately following termination of life support or at external sites following an accident where cardiac arrest has occurred.